Testosterone, as most people know, is the primary male sex hormone, and is associated with many characteristics which are classically considered to be masculine, among these is a predilection toward aggression. A recent study, led by Dr. Justin Carré of Nipissing University and published in the journal Biological Psychology, provides neurological evidence of the mechanism by which Testosterone exerts its influence upon aggression.

Other, earlier research discovered that Testosterone administration affects neurological function, but prior studies were performed on female patients. In those studies, women were provided a one-time dose of Testosterone to monitor the way that it impacted neurological expression.

The goal of Carré's study was to investigate how such Testosterone administration would affect male neurological expression, in comparison to its effects upon the female brain. In particular, Carré wanted to examine how the male brain responds to threats when Testosterone Levels are artificially enhanced in healthy males.

Testosterone, Threat Perception, and the Human Brain

In particular, the researchers placed special emphasis on parts of the brain the are associated with aggression and threat response, such as the periaqueductal gray, the hypothalamus, and the amygdala.

This study involved sixteen patients, all male, and with no signs of illness. They underwent two evaluations, in which they were either provided placebo or a dose of Testosterone. In order to limit the background noise caused by their natural Testosterone Production, all patients were given a medication which dropped their natural Testosterone Production. This also acted as a control mechanism, leading all sixteen patients to have very similar Testosterone Levels prior to experimentation.

The dose of Testosterone given to the participants was only enough to restore physiologically normal Testosterone Levels, in order to evaluate the effect of clinically normal Testosterone Levels upon fear and threat response.

After the participant received their dose of placebo or Testosterone, they entered an MRI machine and performed a face-matching test while their brains were monitored. In this test, the subjects were evaluated in regard to how they perceived a sense of threat in the faces they were presented with.

Testosterone Increases Sensitivity to Perceived Threats

When the MRI data was examined, the results showed that men that were given Testosterone responded to angry faces with heightened activity in all three areas most associated with threat perception—the periaqueductal grey, the hypothalamus, and the amygdala. Testosterone Suppression limited the ability of the placebo patients to perceive threat.

This study represents the first research that definitively links normal Testosterone Levels to the brain's readiness to perceive aggression and threats. Based on this framework, it will be possible to increase our knowledge of how Testosterone affects other aspects of aggression and the way that humans respond to threats, in particular, the way that Testosterone activates and impacts fight-or-flight mechanisms.

Understanding the Psychological Influence of Testosterone

Because Testosterone Therapy is experiencing a continuing surge of popularity, it has increased the importance of studying other aspects of how Testosterone impacts psychology and physiology. Testosterone Injections, Pills, and Creams are commonly prescribed to men suffering from issues related to sexual dysfunction and age-related decline, so the more advanced that our knowledge of the hormone and its functions becomes, the more effectively we will be able to use Bio-Identical Testosterone Treatments in the safest and most effective ways possible.

Testosterone and Aggression Study Discussion

In the last ten years, new technology that allows patients to both view and respond to data in an MRI machine has led to great advancements in our scientific knowledge. In this case, it allows researchers to evaluate how human beings respond to threats at a neurological level, and how Testosterone impacts that threat response.

This study provides distinct evidence that Testosterone leads to a significant increase in activity in the key parts of the brain that control aggression. Testosterone, like many other hormones, plays a role in the way that humans (and males of other species) respond to each other. Testosterone amplifies the brains reactivity to threatening facial expressions made by other males.

Testosterone and the Amygdala

The particular area of the amygdala that responded most significantly is known as the centromedial region. The central nucleus of the amygdala is an important threat processing center which elevates physical and psychological response to aggression via its links to cholinergic neurons in the basal forebrain, brain stem, and hypothalamus. Other researchers have hypothesized that Testosterone production increases activity in the medial amygdala, which directly leads to an increase in aggressive perception and response.

Animal research supports this notion. In studies involving cats, the artificial activation of the medial amygdala leads directly to an increase in highly aggressive behavior. Stimulation of this area leads directly to the stimulation of areas of the Periaqueductal Grey and the Hypothalamus associated with aggression.

Amygdala Sensitive to Sex Hormones

Both of these areas have receptors which respond to both estrogenic and androgenic hormones, meaning that the amygdala conceives of aggression and the Periaqueductal Grey and the Hypothalamus respond to those signals in direct correlation with the activity of sex hormones in the brain. This is the mechanism by which a person sees a threat, and is conditioned to respond to it.

Most similar research with regard to this study has involved the effects of Testosterone on threat perception in women. Also, these earlier studies were more concerned with how Testosterone impacts threat perception four hours or more after Testosterone is administered.

This study, on the other hand, examined the effects of Testosterone (and lack of Testosterone) on threat perception after a shorter period of time, with participants being evaluated ninety minutes after they were provided Testosterone or placebo. Whereas certain aspects of Testosterone do take four hours or more to become apparent, the effect upon the amygdala takes place more rapidly, in 45 minutes or less.

How Could Different Testosterone Levels Impact Threat Perception?

The results of this study do provide some strong evidence with regard to how Testosterone impacts social interaction and aggression, but there is room for further clarification. For example, in this study, Testosterone Gel Treatment was only intended to boost Testosterone Levels to what is clinically considered to be the low end of the normal range.

In the previous studies which were conducted on female patients, Testosterone Levels were increased to a greater level. Based on what we know about Testosterone and the male brain, one could hypothesize that a higher dose of Testosterone would increase threat response to an even greater degree, and this would be an excellent future study.

Although this particular Testosterone study does differ from its predecessor in terms of Testosterone dosage, all of these studies show that Testosterone encourages activity in parts of the brain associated with threat assessment and fear. It will be important to see how different levels of Testosterone impact threat response. It will add to the current body of knowledge and provide a more exact context for how Testosterone impacts psychological and physiological response as it is related to fear.

Testosterone Did Not Affect Neurological Response to Less Threatening Expressions

Testosterone appears to affect threat response only when the face perceived by the subject reflected anger and aggression. Testosterone did not cause spikes in activity in threat-associated brain regions when the subjects looked at pictures of scared or surprised individuals. In this particular study, threat response was associated with an increase in activity in the area of the Hypothalamus associated with fear.

At this point, although it is clear that activity in these areas are associated with Testosterone and the presence of fearful stimuli, the research conducted does not provide any useful evidence regarding whether aggression-response is directly the result of these visual and neurological cues or if there is some other mediating factor that also plays a role.

This study clearly shows that increasing Testosterone Levels boosts neural activity in areas associated with threat response, but further research will have to be conducted in order to assess the level of aggression evoked by a threat stimulus mediated by elevated Testosterone Levels. With this, it will be possible to compare the relative activity of these threat-associated areas of the brain with corresponding feelings of aggression by the participants. Future research could take many of the protocols used in this study in order to assess neurological responses to other stimuli that are affected by Testosterone Levels, including cooperation and risk-taking.

Hormones and Psychology

Although we often like to think of our mind and our body as separate units, they actually function in synchronicity, and hormone balance plays an important role in maintaining physiological and psychological health. Here, we see how the presence or absence of Testosterone can significantly impact the way that our minds respond to aggressive stimuli.

Other research has shown how Testosterone is associated with a number of other factors, including libido, risk-taking, and financial success. As the body of research continues to grow, we will learn more about the underlying processes that allow Testosterone to contribute to better health and the expression of masculinity. One thing is for certain. The world would be a completely different place without Testosterone!

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Disclaimer: The U.S. Food and Drug Administration has voiced caution over the use of testosterone products, citing studies linking them to increased risks of cardiovascular issues. The information on this website is not meant to provide advice or diagnosis, nor is it meant to replace interactive doctor-patient dialogue and diagnosis. Please always speak with your own primary care physicians concerning consultation regarding the starting of therapeutic programs. Agreeing to blood testing analysis does not necessarily equate to a genuine medical or clinic need for therapeutic treatment programs. The statements on this website have not been evaluated by the FDA.

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